This invention relates to the manufacture of an antistatic ball and its use as flotation in a cover of a tank holding flammable liquid.
Balls, particularly light weight balls of plastic, are employed in numerous situations ranging from children's toys to flotation in industrial equipment. Hollow balls, be they of thin walled metallic or plastic material, have excellent flotation characteristics. Plastic balls are preferred for flotation due to ease of manufacture and resistance to corrosion.
A situation of considerable interest in the use of flotation in the construction of tanks containing flammable or explosive liquid, such as an oil tank in a tank farm as found in the petrochemical industry. One form of construction of such a tank is the provision of a circular cylindrical sidewall open at the top, and being closed off by a cover which floats on liquid contained within the tank. The cover is constructed with pontoons which enable the cover to float. It is well known that such tanks may require maintenance after an extended period of use, particularly with respect to insuring the integrity of the flotation employed in the cover. Access ports, large enough to admit entry of a person, are provided at the top of each of the pontoons to an able the person to enter the pontoon to inspect the pontoon.
Generally, the cover is fabricated of metal and has a hollow chamber divided by walls into an array of pontoons to provide sufficient flotation to carry the weight of the cover plus additional weight, such as the weight of snow which might be on the cover. In older oil tank equipment, the cover was constructed of a metal plate with the pontoons mounted beneath the cover plate, while modem tanks have the pontoons located above the metal cover plate. Repairs to the cover may require welding equipment which can be used only after the tank has been taken out of service so as to insure that the cover is clean and that there are no flammable vapors present. If any flammable vapors be present during repair work on the cover, such as the repair of a pontoon of the cover, a spark from the welding may ignite an explosive burning of the vapor.
Repairs may be made also without taking the tank out of service. For example, one of the pontoons, may have sustained a relatively small opening through which liquid can seep resulting in a loss of buoyancy. By means of the access port a person may enter the pontoon and apply foamed urethane plastic as a liquid which later hardens to maintain buoyancy. Use of the plastic is not intended as a permanent repair because the plastic may become impregnated with the flammable liquid. Also, the plastic is disadvantageous because, at the conclusion of the service interval when reconditioning is mandatory, it is very difficult to remove the plastic so as to be able to clean the cover and make any permanent repairs. Clearly, welding cannot be employed for repair until all liquid and liquid soaked flotation, such as the foamed plastic, has been removed.
As an alternative procedure of repair, one might consider insertion in the pontoons of hollow non-foamed plastic bodies to provide sufficient buoyancy so that it is not necessary to repair the leak in the pontoon. However, the use of a plastic hollow body, such as a hollow ball, has been avoided in the petrochemical industry because such a plastic body is electrically insulating and susceptible to developing a static electric charge. There is a danger that the flotation body may suddenly discharge via a spark which can ignite an explosion.